The present invention relates generally to a liquid level indicating system and, more particularly, to such a system for monitoring liquid level variations produced by consumption of the liquid substantially independent of relatively rapid variations in the liquid level due to unavoidable transient disturbances. The system is particularly applicable to measuring the fuel level in a motor vehicle fuel tank and accordingly will be described with reference to this application herein.
Fuel gauge systems have been provided on motor vehicles for many years. A basic gauge system includes a liquid level sensor in the fuel tank and a gauge connected to indicate to the driver the status of the sensor unit and therefor the fuel level. Unfortunately, transient variations of the fuel level, referred to herein as fuel "slosh", are created by the normal operation of the vehicle and interfere with accurate fuel level registration by the gauge. Accordingly, a variety of damping arrangements have been incorporated into the gauge systems to eliminate rapid transient movements of the fuel gauge due to fuel slosh so that the driver can accurately monitor vehicle fuel level. Damping arrangements include mechanical damping devices associated with the level sensor in the fuel tank, the fuel level gauge or both. More commonly in modern gauge systems, electrical damping circuitry is provided.
Examples of prior art damping arrangements include, U.S. Pat. No. 3,886,518 which discloses a tilt switch for bypassing a magnetic fuel gauge under conditions leading to fuel slosh. U.S. Pat. No. 4,635,043 discloses a fuel gauge wherein a fuel level dependent voltage is fed through an integrator circuit having a suitable time constant to transform input voltage surges due to fuel slosh into a slowly changing output voltage which is used to drive the fuel gauge. The integrator circuit is temporarily bypassed to initialize the fuel gauge. In U.S. Pat. No. 4,760,736, the signal from a level sensor is passed through a low pass RC filter to a display in a vehicle. A resistor of the RC filter is temporarily bypassed to initialize the display.
Problems with stability, calibration, repeatable performance and the like lead to the replacement of mechanical devices with electrical damping circuitry. Unfortunately, many of the electrical damping circuits Provided to this time are relatively complicated and hence expensive to produce and service. In addition, subtle errors are introduced by conventional RC type timing circuits such that the damping characteristics change dependent upon the history of the input signal.
Accordingly, an improved liquid level indicating system is needed which will provide a consistent damping characteristic and be simple and inexpensive to produce and service. Preferably, the improved system will facilitate production of the damping circuitry as an integrated circuit to still further reduce costs, ease production and service, and add to the versatility of the system.